1. Field of the Invention
The present invention relates to a current limiting circuit, and especially relates to a current limiting circuit having a metal-oxide-semiconductor filed-effect transistor (MOSFET) for reducing power loss.
2. Description of the Related Art
With reference to FIG. 3, a conventional current limiting circuit has an input positive terminal I/P4, an input negative terminal I/P5, an output positive terminal O/P4, an output negative terminal O/P5, a MOSFET Q4, a current limiting resistor Rlimit, an error amplifier 50, and a PNP common collector amplifier 60. A drain of the MOSFET Q4 is electronically connected to the input positive terminal I/P4, a source of the MOSFET Q4 is electronically connected through the current limiting resistor Rlimit to be grounded, and a gate of the MOSFET Q4 is electronically connected to an output terminal O/P6 of the PNP common collector amplifier 60. The error amplifier 50 has a positive input, a negative input, and an output. A voltage dividing circuit 51 is electronically connected between a reference voltage source Vref and the output negative terminal O/P5, and has a first resistor R4 and a second resistor R5. The first resistor R4 and the second resistor R5 are electronically connected in series, and a node between the first resistor R4 and the second resistor R5 is electronically connected to the positive input of the error amplifier 50. The negative input of the error amplifier 50 is grounded, and the output of the error amplifier 50 is electronically connected to an input terminal I/P6 of the PNP common collector amplifier 60. The input negative terminal I/P5 is electronically connected to the output negative terminal O/P5, and the output positive terminal O/P4 is grounded.
When the conventional current limiting circuit is operated, the input positive terminal I/P4 and the input negative terminal I/P5 are connected to a power source 30, and the output positive terminal O/P4 and the output negative terminal O/P5 are connected to a load 40. A drain current of the MOSFET Q4 is controlled by a gate voltage of the MOSFET Q4. The gate voltage of the MOSFET Q4 is an output voltage of the PNP common collector amplifier 60, and an input voltage of the PNP common collector amplifier 60 is an output voltage of the error amplifier 50. The drain current of the MOSFET Q4 flows through the current limiting resistor Rlimit and the load 40.
When the drain current of the MOSFET Q4 suddenly rises up, a voltage through the current limiting resistor Rlimit also rises up. One end of the current limiting resistor Rlimit is grounded, and the other end of the current limiting resistor Rlimit is electronically connected to the source of the MOSFET Q4. Further, a voltage of the source of the MOSFET Q4 also rises up, but the rising drain current of the MOSFET Q4 does not cause the voltage of the gate of the MOSFET Q4 to rise up. Then, a gate-to-source voltage of the MOSFET Q4 decreases. A maximum of the drain current is limited by the gate-to-source voltage of the MOSFET Q4. When the gate-to-source voltage of the MOSFET Q4 decreases, the maximum of the drain current also decreases. Therefore, the maximum of the drain current of the MOSFET Q4 can be decreased by the current limiting resistor Rlimit. Then the limited drain current of the MOSFET Q4 can protect the MOSFET Q4 from damages when the drain current of the MOSFET Q4 suddenly rises up.
The current limiting resistor Rlimit is mounted on a power transmission route of the conventional current limiting circuit. When the conventional current limiting circuit is in normal use, the current limiting resistor Rlimit has the drain current flowing through, and causes power consumption. Therefore, the conventional current limiting circuit needs to be further improved for reducing the power consumption under normal use.